Fluid catalytic cracking (FCC) is one of the most important conversion processes used in petroleum refineries. It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefinic gases, and other products. FCC is a vital unit in refinery operations due to its impact on overall refinery profitability. Due to its importance, operation of the FCC is shifting to meet changing market demands for particular fuel specifications, such as to maximize particular products, such as diesel fuel, LPG, propylene etc.
In the conventional process of propylene and LPG recovery from FCC main column overhead product mixture, the gaseous fraction from the main fractionator condenser/separator is fed to a two stage compressor. The first stage discharge is partially condensed and cooled in inter-stage coolers. The resulting liquid and gaseous fractions are separated in inter-stage receiver. Second stage compressor discharge after combining with the liquid fraction from first-stage receiver is condensed and cooled in second stage high pressure condenser/coolers and received in a high pressure receiver cum separator.
The liquid fraction from high pressure receiver is fed to a de-ethanizer column or C2− stripper where ethane, ethylene and lighter material present in the feed are removed. The overhead vapours from stripper are recycled back to high pressure receiver via high pressure coolers. Bottom product of the stripper is fed to a debutanizer column where propylene is obtained as a part of the overhead product and the bottoms product thus obtained is referred to as stabilized naphtha.
The gaseous fraction from high pressure receiver is supplied to an absorber. In the absorber, C3-C4 components present in the gaseous feed are preferentially absorbed by an absorber fluid also referred to as absorber oil or lean oil. Overhead liquid from the main fractionator (typically known as unstable naphtha) and debut bottoms liquid (typically known as stabilized naphtha) are commonly used as absorber oil. Typical temperature of lean oil supplied to the absorber column is between 30 and 40° C. Side coolers are provided to remove heat of absorption from absorber oil. Rich absorber oil from absorber bottom is cooled and supplied to high pressure receiver from where it is fed to de-ethanizer column. Absorber overhead gases are further treated to recover any gasoline range material still present in the gas leaving the absorber.
Due to the presence of significant quantity (>5 mol %) of propylene present in the unstabilized naphtha stream, the recovery of propylene is limited up to 97% in the conventional process. Presence of significant quantity (>5 mol %) of propylene in the unstabilized naphtha stream results in significant reduction in mass transfer of propylene from fuel gas to absorber oil.
WO2000031214A2 describes a process for the recovery of an olefin from a gas feed stream comprising hydrocarbons and minor amounts of olefins. The process comprises introducing the gas feed stream into a single recovery distillation column; withdrawing an overhead vapor stream from said column; cooling and partially condensing the overhead vapor stream in the overhead condenser; phase-separating the overhead vapor-liquid in the reflux drum; withdrawing from the reflux drum, an olefin-lean, overhead vapor stream and recycling the separated liquid from the reflux drum to the column; withdrawing from the column, a liquid C3+, C4+, C5+, or C6+ bottoms product stream; recycling a portion of the liquid bottoms product stream to the overhead condenser or the upper tray section of said column; and withdrawing an olefin-rich, vapor phase, side product stream rich in a selected olefin.
U.S. Pat. No. 7,074,323 B2 describes a process to debottleneck the conventional process for gas concentration unit wherein unstabilized naphtha, a liquid fraction obtained by cooling the main fractionator overheads and subsequently separating the obtained gaseous and liquid fractions, is separated by distillation into a heavy boiling fraction (initial boiling point 100-160° C.) and a lighter fraction (final boiling point 10-160° C.). The lighter fraction after being cooled between 8 to about 25° C. is fed to the absorber while the heavier fraction is directly fed to the debutanizer. This reduces liquid and gas loads on absorber, stripper and debutanizer. However, the recovery of propylene is not much improved since the lighter fraction contains in fact higher percentage of propylene than the original cut before fractionation. Besides this the main objective of U.S. Pat. No. 7,074,323 B2 is to reduce the load on C2 stripper and debutanizer section rather than improving propylene recovery.
U.S. Pat. No. 3,893,905 describes a process wherein a hydrocarbon feed contacts cracking catalyst at cracking conditions in a reaction zone, reaction products are withdrawn from the reaction zone and introduced into a fractionation zone, overhead vapors from the fractionation zone are passed to a condenser, and effluent from the condenser passes in separate conduits to gas concentration facilities, the improvement comprises: condensing overhead vapors in a differential condenser and in the differential condenser separating overhead vapor into a gaseous phase containing propylene and a liquid phase comprising hydrocarbon molecules having four or more carbon atoms, separation is effected by withdrawing liquid phase upon condensation to form gaseous and liquid phases containing a non-equilibrium distribution of propylene.
Although recycling of downstream naphtha or a heavier hydrocarbon fraction of FCC has been to an upstream section in U.S. Pat. No. 5,846,403 and incorporation of extra columns for further treating the naphtha is disclosed in US20020003103A1; EP142900A2; U.S. Pat. No. 5,846,403; Oil & Gas Journal 101, 52-53, 56-58, 2003. The extra column(s) for treating the (recycled) naphtha is found to be a reactor column for further cracking of the naphtha in order to increase the propylene yield, and not for increasing propylene recovery in the gas concentration section by stripping the naphtha in the extra column.
The concept of increasing absorptive capacity of naphtha or heavier condensate stream from the main reactor for C3, C4 in the gas concentration section by cooling of the heavier condensate to a deep sub-zero temperature is disclosed in WO2000031214A2. Besides this, the concept of pre-empting mixing of C3, C4 with the heavier condensate after the main reactor by incorporating differential condenser is disclosed in U.S. Pat. No. 3,893,905.
As already mentioned in the conventional process, the product mixture from FCC main column overhead comprising naphtha, LPG and fuel gas, are first condensed and gravity separated to produce unstabilized naphtha, which is subsequently used, along with debutanizer naphtha, to absorb propylene and LPG from fuel gas. One of the approaches to enhance propylene recovery beyond 97% is by recycling more of debutanized bottoms viz., stabilized naphtha in combination with the unstabilized naphtha to the absorber. This approach, after a limited success within the design limits, ultimately hits vapor-liquid flooding or reboiler/cooling duty limits in any of the C2 stripper, debutanizer and the absorber column. This requires providing higher gas-liquid capacities and reboiler duties in new designs or by debottlenecking the existing the absorber, de-ethanizer or debutanizer columns constrained due to vapor/liquid flooding or due to limited reboiling duties. Thus, disproportionately large column capacity and reboiler duties need to be provided in the new designs or through revamp of existing units in this approach.
Another approach as followed in the process disclosed in Indian Patent Application No. 1570/MUM/2009 achieves this objective by supplying lean absorber oil generated by stripping off C4 and lighter components from unstabilized naphtha in a separate column. Though this process improves propylene and LPG recovery with no additional recycle of debutanizer naphtha, it requires additional compression capacity, condensers capacity and reboiler duty for recycling the stripped off lighters.
Still another approach is to minimize or completely withdraw the unsaturated naphtha from the primary absorber since unsaturated naphtha is substantially rich in propylene and LPG content and to supply only the debutanizer naphtha at higher flows to the absorber to enhance absorption of propylene. The unsaturated naphtha is directly fed to high pressure receiver or C2 Stripper; additional capacity and associated reboiler duties for each of the existing de-ethanizer and debutanizer needs to be provided in this case to accommodate increased gas-liquid flows, though no additional column for fractionation is required. No additional wet gas compression capacity is required.
The processes possible for achieving same improvement in propylene and LPG recovery are summarized below:    1. Debottleneck/design to provide additional capacity for each of the absorber, de-ethanizer and debutanizer to accommodate increased gas-liquid capacities and associated reboiler duties for increasing recycle flow of Debutanizer naphtha (along with unsaturated naphtha) to the Primary Absorber.    2. Providing an additional column and reboiler duty for stripping the lighters from unstabilized naphtha to supply additional lean absorber oil (along with the debutanizer naphtha) to primary absorber. The stripped off lighters which are cooled and recycled back requiring additional wet gas compressor capacity.    3. Feeding the unsaturated naphtha directly to high pressure receiver or C2 stripper bypassing the absorber as disclosed in the present invention. Only debutanizer naphtha at higher flows is supplied to the absorber; additional capacity and associated reboiler duties for each of the de-ethanizer and debutanizer needs to be provided in revamp/new design to accommodate increased gas-liquid flows. However, no additional column for fractionation or no additional wet gas compression is required.
The process configuration as disclosed in prior art requires minimum energy and minimum tray area but requires additional wet gas compressor capacity. For the wet gas compressor constrained units, the process disclosed in the present invention is efficient in terms of energy and total distillation area requirements for enhancing the recovery of propylene and LPG from fuel gas beyond 97 wt %.